1. Field of the Invention
The present invention relates to a temperature compensation-type balance, a timepiece movement, a mechanical timepiece and a manufacturing method of the temperature compensation-type balance.
2. Description of the Related Art
A speed regulator for a mechanical timepiece is generally configured to have a balance and a hairspring. Such a balance is a member which oscillates by cyclically rotating forward and backward around an axle of a balance staff, and it is important that an oscillation cycle thereof is set within a predetermined control value. This is because a rate of the mechanical timepiece (degree indicating whether the timepiece is fast or slow) varies if the oscillation cycle is beyond the control value. However, the oscillation cycle is likely to vary due to various causes, and for example, also varies due to a temperature change.
Here, an oscillation cycle T described above is expressed by the following Equation (1).
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      T        =                  2          ⁢          π          ⁢                                    I              K                                                          (        1        )            
In Equation (1), the “moment of inertia of the balance” is indicated by I and a “spring constant of the hairspring” is indicated by K. Therefore, if the moment of inertia of the balance or the spring constant of the hairspring varies, the oscillation cycle also varies.
Here, a metal material used in the balance generally includes a material whose linear expansion coefficient is positive and which is expanded due to a temperature rise. Therefore, the balance wheel is radially enlarged to increase the moment of inertia. In addition, since the Young's modulus of a steel material which is generally used in the hairspring has a negative temperature coefficient, the temperature rise causes the spring constant to be lowered.
As described above, in a case of the temperature rise, the moment of inertia is increased accordingly and the spring constant of the hairspring is lowered. Therefore, as is apparent from Equation (1) described above, the oscillation cycle of the balance has characteristics of being shorter at a low temperature and being longer at a high temperature. For that reason, as temperature characteristics of the timepiece, the timepiece is fast at the low temperature and slow at the high temperature.
Therefore, as a measure to improve the temperature characteristics of the oscillation cycle of the balance, the following two methods have been known.
As the first method, there has been a known method where, in place of causing the balance wheel to be in a circle shape of a completely closed loop, the balance wheel is divided across two places in a circumferential direction to be arc-shaped portions, and each of the arc-shaped portions is formed of a bimetal where metal plates made of materials with a thermal expansion coefficient different from each other are radially bonded together, thereby setting the arc-shaped portions of which one end portion in the circumferential direction is a fixed end and the other end portion in the circumferential direction is a free end (refer to JP-B-43-26014 (Patent Reference 1)).
Generally, as described above, the balance wheel is radially enlarged due to thermal expansion along with a temperature rise, thereby increasing the effective moment of inertia. However, according to the first method, at the time of the temperature rise, the arc-shaped portions made of the bimetal are deformed inward so as to move the free end side radially inward due to a difference in the thermal expansion coefficient. This enables an average diameter of the balance wheel to be radially reduced and enables the effective moment of inertia to be lowered. Thus, it is possible to cause the temperature characteristics of the moment of inertia to have a negative slope. As a result, it is possible to change the moment of inertial to the extent of counter-balancing temperature dependence of the hairspring, thereby enabling the temperature dependence of the oscillation cycle of the balance to be lessened.
The second method is a method where a temperature coefficient of the Young's modulus near an operating temperature range (for example, 23° C.±15° C.) of the timepiece is caused to have positive characteristics by employing a constant elastic material such as Coelinvar as a material of the hairspring.
According to this second method, in the operating temperature range, it is possible to cancel the change in the moment of inertia of the balance with respect to the temperature by counter-balancing the linear expansion coefficient of the balance wheel and the linear expansion coefficient of the hairspring, thereby enabling the temperature dependence of the oscillation cycle of the balance to be lessened.
Incidentally, in the above-described first method, the arc-shaped portions made of the bimetal are formed by bonding the metal plates radially inward and the metal plates radially outward having a thermal expansion coefficient different from each other, and the bonding method can be exemplified such as brazing and crimping. However, in these methods, finishing depends on a bonding condition and the like thereat so that it is difficult to ensure constant form precision. Moreover, since the arc-shaped portions are configured of two metal plates, when performing the brazing and the crimping, or forming each of the arc-shaped portions by cutting, there is a possibility that two metal plates may be elastically deformed.
Due to these reasons, it is difficult for the arc-shaped portions made of the bimetal to be finished with the high form precision, and thus, adjusting the moment of inertial and setting a degree of temperature compensation are likely to be unstable. Additionally, an iron based material such as invar (low thermal expansion material) is generally employed as the material of the metal plate disposed radially inward, and this leads to a problem of generating rust unless plating and the like are not performed. Therefore, manufacturing needs labor, thereby leaving room for improvement.
In addition, in the above-described second method, there is a possibility that when manufacturing the hairspring using a constant elastic material such as Coelinvar, a temperature coefficient of the Young's modulus may vary greatly depending on composition during a melting process and various processing conditions during a heat treatment process or the like. Therefore, a strict manufacturing control process is required, thereby not facilitating the production of the hairspring. Accordingly, in some cases, it is difficult to cause the temperature coefficient of the Young's modulus to be positive near the operating temperature range of the timepiece.